An archaic or variant form of diligence; careful and attentive effort or application in one's work or duties.
From Latin diligentia via Old French, with -cy as an alternative suffix for creating abstract nouns. This form appears in older English texts before 'diligence' became the standard.
The -cy suffix in 'diligency' is the same one you see in 'fluency,' 'agency,' and 'currency'—it's an older way of forming nouns that sometimes gets replaced by -ence or -ance!
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